The main objective of this study is to determine if reduced energy expenditure is a marker for those children who will become obese. A group of 120 prepubertal children with equal numbers blacks and whites, and an equal number of girls (9-10 yr) and boys (10-11 yr) will be studied initially, and again after 2 years. There will be 4 groups of children based on skinfold measurements: 30 lean (lowest quintile of subscapular plus triceps), 30 with central fat (highest quintile of subscapular/triceps ratio), 30 obese (highest quintile of subscapular plus triceps) and 30 with peripheral fat (lowest quintile of subscapular to triceps ratio). Several components of energy expenditure will be measured or calculated: 1. Total daily energy expenditure (TDEE) by doubly labeled water; 2. Resting metabolic rate (RMR); 3. The thermic effect of food (TEF) by respiratory gas analysis; 5. Daily physical activity will be calculated by subtracting RMR and TEF from TDEE. Daily energy intake will be calculated from TDEE adjusting for changes in body composition. The activity of the sympathetic nervous system will also be assessed by several methods. First, urinary catecholamine excretion in response to a meal will be examined. Second, timed urinary catecholamine excretion will be determined during an isometric hand grip exercise and third, catecholamine excretion will be measured following a change in posture from supine to standing. The components of energy expenditure and measures of sympathetic activity will be compared between the 4 groups initially and after 2 years. The various measures of energy expenditure and sympathetic activity will be correlated with body composition between the 4 groups and more importantly, the change in body composition over the two year period. From these analyses, we will be able to determine if total daily energy expenditure, resting metabolic rate, the thermic affect of food, daily physical activity and daily intake are markers for obesity in this population of children going through puberty. In addition activity of the sympathetic nervous system, as measured by urinary catecholamine excretion, will be correlated with the various components of energy expenditure. Therefore, we will be able to determine if the sympathetic nervous system is impaired, causing a decrease in energy expenditure which leads to obesity.